I have a new custom steel frame. The seat post is OD: 27.2mm
The seat tube is OD 28.6mm

So I bought a 28.6mm seatpost clamp.. and my seat post doesn't fit through the ridge of the clamp, but it fits fine over the seat tube. Seatpost clamps appear to only be sold according to the major diameter (OD of seat tube).

I have this diameter and it's correct (28.6), so how do I find one that will fit my seatpost (27.2)?

Edit: Thanks guys, I think I'll file it down and keep my eyes open for the right diameter as well.